This invention relates to a means of measuring the degree of similarity between two images. Specifically, it relates to a method of aligning and verifying the alignment of images such as radiotherapy images used in radiotherapy treatment systems. More specifically, the present invention is directed toward an improved method and apparatus for processing and comparing a reference image to one or more portal fields of radiation using Fourier correlation techniques.
Radiotherapy treatment is generally used to supply dosages of radiation upon isolated portions of a patient's body. Dosages of radiation are often used in the treatment of cancer. However, when supplying radiation upon a patient's body, it is crucial that the cancerous portion be isolated, and radiation be supplied only toward that portion. With modern radiotherapy equipment, carefully collimated radiation beams (called treatment portals) can irradiate localized cancerous tissue while sparing much of the surrounding normal tissue. Conventional radiotherapy consists of about 30 consecutive daily treatments using one or two portals each day. Repositioning of the patient is critically important for success of therapy. Therefore, it is important that during each radiation treatment session, the treatment portals be aligned consistently upon the cancerous tissue. By fixing the radiation portals upon a specific portion of the patient's body during each treatment session, the radiotherapist is ensured that maximum effective treatment of the patient's cancerous tissue will be maintained while minimizing radiation exposure to normal tissue.
Conventional alignment systems use pre-therapy portal filming to verify and maintain the proper alignment and position of a portal. Verification using pre-therapy filming is severely limited by the time required to develop the film and the logistics of film evaluation and storage. Pre-therapy filming entails too great a time delay between exposure of the film and review of the developed image to allow image evaluation before irradiation of the patient. Current practice is to delay review of the portal film until after the filmed therapy session and before the next therapy session. The time and expense required to acquire an image on conventional x-ray film also precludes daily evaluation of portal alignment. In an effort to overcome these problems, recent advances in the technology of high resolution detection of energetic radiation have made practical on-line electronic portal imaging devices (EPIDs). EPIDs make it possible to record and evaluate images on-line and prior to the delivery of each radiation dose. On-line monitoring allows a patient to be repositioned in accordance with the radiation portal and during radiation treatment.
Using EPIDs, one may test the sequence of images, f.sub.test, acquired during a course of therapy against a standard image, f.sub.ref. A reference image is identified which outlines a fairly precise location for which radiation treatments are desired. The reference image may be taken on-line with the EPID at the beginning of the treatment process, and the resulting reference image stored for subsequent use and comparison with portal images during future radiation treatment sessions. Alternatively, the reference image may be a digitally reconstructed radiograph computed from a serial sequence of CT scans of the patient or else it may also be derived from a simulation radiograph. The major problem with EPIDs is that comparison of reference images with subsequent radiation treatment portals is time consuming and requires the presence of the radiotherapist and/or communication of detailed acceptance criteria to trained technologists. Images conventionally consist of approximately 512.times.512 arrays of pixels of which each pixel can have a plurality of eight or twelve bits to encode gray levels. Thus it is possible to compare images numerically. However, it is difficult to ascertain what algorithm should be used for the comparison. Observation of the subtraction image, f.sub.test -f.sub.ref, provides a qualitative means of assessing portal alignment. But subtraction images provide no universal quantitative scale for measuring a degree of verification. Automated identification of specific and anatomical structures is not presently feasible with minicomputers. Another method is to superpose a graticule or scale generated by the computer over the image. Although quantitative assessment may be made of identifiable features which intersect the scale, this method still gives no global measure of the degree of similarity between two images.